This invention relates to a new novel CRT device for generating a bright blue light spot of the type that is particularly useful for projection color television and information display.
Blue light radiation for use in projection color television generally is produced by the electronic bombardment of a blue luminescent zinc sulfide phosphor, for example a silver-activated zinc sulfide phosphor (P-22 blue). This phosphor, when excited by electronic bombardment produces a high amount of the desired radiation at 460 nm as well as significant amounts of undesired radiation in the 432-500 nm zone surrounding the desired main peak at 460 nm.
As part of this undesired radition is in the green zone, there is some dilution of the desired blue radiation. Further due to the presence of the high degree of radiation surrounding the desired frequency at 460 nm blurring of the image may occur when the 460 nm radiation image is brought into focus, due to the chromatic abberation of the lens system.
Various means have been proposed for filtering out the undesired radiation for color television tubes. For example, Denki, Japanese Pat. No. 57-180859 shows the use of a glass filter plate containing Nd.sub.2 O.sub.3 and a small amount of either Cr.sub.2 O.sub.3 or Pr.sub.2 O.sub.3. Seward et al U.S. Pat. No. 4,086,089 employs glass faceplates for color television tubes which faceplates function as filters. These faceplates contain Na.sub.2 O, AgHal and SiO.sub.2. In addition, Dutch Octrooi 144063 shows a solid optical filter employing a lanthanum salt or oxide. However, none of these patents show a filtering means capable of substantially reducing the undesired radiation or wings surrounding the desired 460 nm radiation produced in a cathode ray tube containing a silver-activated zinc sulfide phosphor without significantly reducing the 460 nm radiation.